The present invention relates to systems for launching a launch vehicle from an aircraft in general, and to vehicles that are air launched captive-on-bottom from a carrier aircraft, in particular.
Rocket launch vehicles, both with and without crews, have conventionally been launched from ground-based installations near coastal areas. These installations have been fixed launch stands, such as Cape Canaveral, or specialized mobile carriers used for missile launches. Ground launches, however, present problems of launch delays due to inclement weather, the necessity to clear the vicinity of air traffic to avoid collision, and in particular present concerns about the overflight of populated land areas in the flight path of the launch vehicle. Moreover, crew safety equipment for manned vehicles must take into account the requirement to remove the crew from a vehicle in the case of a launch abort or failure which may occur on the ground or very close to the ground.
Some of the problems of ground launching can be overcome by launching the rocket launch vehicle from an aircraft in flight. An air launch offers several advantages over a ground launch, such as the avoidance of weather related delays, the simplification of operations, and increases in safety, both for the crew by simplifying abort options, and for the public because of the ability to avoid the overflight of populated areas. In addition, air launching presents design options that simplify the operation of the launch vehicle engine. In particular, it is known that high area ratio nozzles for a given engine pressure increase performance. Thus if a lower engine pressure is used to take advantage of the high area ratio nozzle, a lower cost solution is possible. A lower pressure engine can be pressured fed which means that no turbo-pumps or gas generators are needed, resulting in a less complex solution. A lower pressure engine is also a safer solution because no operational pressure fed rocket has ever exploded.
There are several methods of air-launching, each with their own pros and cons. One method is captive-on-top, where the launch vehicle is carried on top of the carrier aircraft. While a large launch vehicle can be carried, wings are required for separation from the carrier aircraft, and mating the launch vehicle to the carrier aircraft requires a large mating structure which needs to be available at every staging site from which the carrier aircraft is launched. This method was used for flight trials of the US Space Shuttle. Another method is towed, where a winged launch vehicle is pulled to altitude by the carrier aircraft and then released. This method requires lower cost modifications to the carrier aircraft than the captive-on-top method, but there is the additional hazard of a broken towline, and the towed launch vehicle must have wings sized for takeoff. Another method is aerial refueled launching, where the launch vehicle takes off with a smaller and lighter weight propellant load and then is refueled at altitude to continue with the mission. Wings are still required in this approach, although they can be smaller than for a towed vehicle. An example of this would be the SR-71 high-altitude reconnaissance aircraft. Another method is the rockoon, where the launch vehicle is brought to altitude with a balloon. This method, however, must have a calm day for launch and there is greater hazard to objects on the ground from the falling balloon after the release of the launch vehicle. A final method is to carry the launch vehicle either internally or captive-on-bottom of the carrier aircraft. Whether carried inside or underneath, there are limits to the size of the launch vehicle that can be carried. However, an increased launch altitude is possible along with the reduction or elimination of the wings on the launch vehicle. The internal carry approach additionally has a more complicated release mechanism in order to extract the launch vehicle from the carrier aircraft. The air launched, captive-on-bottom vehicle offers a good compromise between vehicle size, payload amount and operational complexity. Examples of air launched, captive-on-bottom vehicles include the Pegasus vehicle developed by Orbital Sciences Corporation, and the SpaceShipOne vehicle developed by Scaled Composites.
While existing air-launched, captive-on-bottom vehicles use different types of carrier aircraft, such as commercial jet, military bomber or custom designed airplane, each of these launch vehicles employs a forward trajectory that carries it in front of its carrier aircraft. Typically during an air launch, a launch vehicle will drop below the carrier aircraft and then re-cross the carrier aircraft's altitude in front of it. Vehicles such as the X-15, the Pegasus rocket, and SpaceShipOne have used this forward crossing trajectory. These vehicles must use wings in order to transition from the horizontal to vertical orientation.
Using wings subjects these vehicles to large longitudinal bending stress during the 2-3 g pull-up maneuver they must do as they transition from horizontal to vertical flight. This high sideways acceleration requires a stronger and heavier fuselage structure. Another disadvantage of the use of wings is the need for greater peak first stage engine thrust vectoring control. This engine thrust vectoring assists in the change of orientation, horizontal to vertical, of the launch vehicle, and helps to maintain stability during this orientation transition.
Because of the additional systems often required with the use of wings, such as control surface actuators, auxiliary power units, and thermal protection, a common design goal is to reduce or eliminate the wings. However, forward crossing trajectories, for launch vehicles without wings, require flight at large angles of attack during the transition from horizontal to vertical flight. This transition segment is a high dynamic pressure segment of the trajectory, which results in large angles of attack at peak dynamic pressures. This additional load requires a stronger fuselage structure, thereby increasing the weight of the launch vehicle and offsetting the weight savings by eliminating the wings in the first place.
Lastly, there is the safety concern during a forward crossing air launch, though minimized through careful planning, of the possibility of falling debris from the launch vehicle hitting the carrier craft, either accidentally or as a result of the launch system's operation.
What is needed is an air launch system that has the advantages of air launching, captive-on-bottom without the negative aspects of a forward-crossing trajectory.